The construction of fast inverters comprising several single elements connected in parallel is only possible with rather large expenditure for control and electronic circuitry. This is only acceptable if the inverter must satisfy certain conditions which cannot be satisfied in another way, and/or if the parallel switching arrangement has certain decisive advantages in comparison with usual solutions comprising only a single element.
It is an aim of present-day builders of motors to produce motors which run ever faster. So-called "fast-running drives" require operational frequencies which exceed 500 Hz. In view of the power aimed at, in the range of several hundred kVA, usual thyristor- or GTO (gate turn off) inverters can at best provide these frequencies in the shape of elementary steps, as shown in FIG. 1.
The drawbacks of this crude, rectangle-shaped voltage and of the non-sinus shaped current curve which it entails, that is additional losses and peaks of the moment are well known. It has therefore been known for a long time to divide these voltage segments into smaller time units, in order to obtain a nearly sinusoidally-shaped curve of the motor current. With respect to the inverter this entails that the power switches must be switched on and off accordingly faster. The faster these switches work, the finer one can make the time slices of the voltage segments, and the more sinusoidally-shaped the motor current becomes, see FIG. 2.
If now the operational frequency of the usual 50 to 120 Hz is raised to over 500 Hz at fast-running drives, then the switching frequency of the switches must also be increased accordingly. This means that the thyristor- and GTO switches come even nearer to the limits of their switching speed.
Fast inverters with switching frequencies up to 100 kHz and able to deliver a sinusoidally-shaped current curve even with 500 Hz output frequency are normally available only up to a power of about 50 kVA. An increase of the power up to several hundred kVA with the same high clock frequency is scarcely imaginable with present-day techniques, hence the desire to use inverters which are connected in parallel.
The design of a parallel connected inverter differs in many ways from that of an inverter built in the usual way. Whilst hitherto the control, switching and the protection of the power rectifiers were the essential issues, general design considerations now come into the foreground. Protection problems cannot be considered as local issues any more, but must be viewed in relation to the overall system. Communication problems between the partial inverters must be solved, as must those which pertain to the reliability of a distributed system. One must never forget that the release time, that is the time available for collecting, processing, and distributing instructions to the entire system may not exceed a few microseconds. In what follows it will therefore be considered how a technically and economically acceptable arrangement of parallel connected inverters may look like.
In what follows different possibilities to connect inverters in parallel will be considered and a systematic description will be attempted. We will distinguish the following four main criteria:
coupling of energy PA1 realisation of the connection on the load-side PA1 timing PA1 regulating of the current. PA1 technical features PA1 construction possibilities PA1 economy PA1 redundancy features. PA1 unsatisfactory dynamic because of the large PA1 coils PA1 limited efficiency PA1 large size of the coils.
The coupling of the energy, i.e. the collection of the energies delivered by the partial inverters, can be performed electrically or magnetically. If one uses an electrical coupling, the different phases of the partial inverters will be connected, and the load will be attached in the usual way. A magnetic coupling is said to exist for instance when using a motor with multiple winding.
In the case of an electrical coupling, the partial inverters can be mutually connected either through large connecting coils which have a function tied to the system as a whole, or through small coils which solely act as protection. This last case is called a direct coupling, see FIG. 4.
The timing relates to the switching times of the power switches. If these are switched in accordance with the same time grid for all partial inverters, this is termed as synchronous timing, and else as asynchronous timing, see FIG. 5.
If each partial inverter controls a current independently of the others, one has a single current regulation, see FIG. 6. A global current regulation is said to exist when the total output current of all partial inverters is measured and controlled, see FIG. 7.
If one combines the above features, one obtains parallel inverters with widely differing characteristics. They differ in particular in the following respects:
The most important possibilities shall now be briefly presented and described.
This way of coupling represents the most simple electrical connection between inverters. All partial inverters are given the same rated value of the current, and then each single one endeavours to make its output current equal to the rated value. However, this may have the result that for instance the first partial inverter closes the upper switch of phase A, whilst the second partial inverter simultaneously closes the lower switch of the same phase. This generates a "hot path" between these two partial inverters and unavoidably destroys the switches. This can only be avoided by inserting sufficiently large inductive coils between the partial inverters and the bus-bar, in order to limit the current increase, see FIG. 8.
The drawbacks of this solution are:
The task of the connecting coils is a limitation of the current increase in the "normal working case" of a "hot path between the partial inverters". However, such coils also limit the largest achievable current increase on the load side.
The smaller the connecting coils get, the stronger is the mutual influence of the partial inverters on each other. This results in a mutual stir up, that is idle power is pushed back and forth between the partial inverters and the efficiency drops accordingly. One must also not underestimate the extremely large size of the necessary coils.
The block diagram of FIG. 9 shows that in this case only the global current is measured. Upon a command of the host computer, and in dependence of a comparison with the rated value, all switches of the different partial inverters are being switched in and out simultaneously and in the same way. This removes the problem of the "hot path" between the different partial inverters, and in theory the connection coils may be discarded or, because they now have only an auxiliary function, they can be made much smaller.
In spite of its simplicity and obviousness, this solution can practically not be implemented. The switches, which are shown as simple lines in FIG. 9, actually consist of many elements, quite apart from their command device and so on. Local distribution and thermal influences can cooperate to produce a strongly asymmetric current distribution between the different partial inverters.